Positive active material for rechargeable lithium battery, preparing method thereof and rechargeable lithium battery including the same

ABSTRACT

Disclosed are a positive active material for a rechargeable lithium battery, a method of preparing the same, and a rechargeable lithium battery including the same. 
     The positive active material includes a first positive active material in a form of secondary particles including a plurality of primary particles that are aggregated together, and a second positive active material having a single crystal form, wherein both of the first positive active material and the second positive active material are nickel-based positive active materials, each of the first positive active material and the second positive active material is coated with cobalt, and a maximum roughness of a surface of the second positive active material is greater than or equal to about 15 nm.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2021-0069175 filed in the Korean IntellectualProperty Office on May 28, 2021, the entire contents of which are herebyincorporated by reference.

BACKGROUND 1. Field

Example embodiments of the present disclosure are related to a positiveactive material for a rechargeable lithium battery, a method ofpreparing the same, and a rechargeable lithium battery including thesame.

2. Description of the Related Art

A portable information device such as a cell phone, a laptop, smartphone, and the like or an electric vehicle has used a rechargeablelithium battery having high energy density and easy portability as adriving power source. Recently, research has been actively conducted touse a rechargeable lithium battery having high energy density as adriving power source or power storage power source for hybrid orelectric vehicles.

Various positive active materials have been investigated to realizerechargeable lithium batteries for application to such uses. Among them,lithium nickel-based oxide, lithium nickel manganese cobalt compositeoxide, lithium nickel cobalt aluminum composite oxide, lithium cobaltoxide, and the like are mainly used as a positive active material.However, these positive active materials have structures that collapseor crack during repeated charge and discharge cycles, and thus, problemsof deteriorating or reducing a long-term cycle-life of a rechargeablelithium battery and increasing resistance and thus not exhibitingsatisfactory capacity characteristics. Accordingly, development of anovel positive active material securing long-term cycle-lifecharacteristics as well as realizing high capacity and high energydensity is being investigated.

SUMMARY

A positive active material for a rechargeable lithium battery havingimproved cycle-life characteristics while implementing a high capacity,a method of preparing the same and a rechargeable lithium batteryincluding the same are provided.

In an embodiment, a positive active material for a rechargeable lithiumbattery includes a first positive active material in a form of secondaryparticles including a plurality of primary particles that are aggregatedtogether, and a second positive active material having a single crystalform, wherein both of the first positive active material and the secondpositive active material are nickel-based positive active materials,each of the first positive active material and the second positiveactive material is coated with cobalt, and a maximum roughness of asurface of the second positive active material is greater than or equalto about 15 nm.

In another embodiment, a method of preparing a positive active materialfor a rechargeable lithium battery includes mixing a first nickel-basedhydroxide and a lithium raw material together and performing a firstheat-treatment to prepare a first nickel-based oxide in a form ofsecondary particles in which a plurality of primary particles isaggregated, mixing a second nickel-based hydroxide and a lithium rawmaterial together and performing a second heat-treatment to prepare asecond nickel-based oxide, and mixing the first nickel-based oxide, thesecond nickel-based oxide in a single crystal form, and a cobaltcompound together and performing a third heat-treatment to coat thefirst nickel-based oxide and the second nickel-based oxide with cobalt,thereby obtaining a final positive active material including the firstpositive active material and the second positive active material.

In another embodiment, a rechargeable lithium battery including apositive electrode including the positive active material, a negativeelectrode, and an electrolyte is provided.

The positive active material for a rechargeable lithium batterymanufactured according to an embodiment and a rechargeable lithiumbattery including the same may exhibit excellent charge and dischargeefficiency and cycle-life characteristics while realizing a highcapacity and high energy density.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, together with the specification, illustrateembodiments of the subject matter of the present disclosure, and,together with the description, serve to explain principles ofembodiments of the subject matter of the present disclosure.

FIG. 1 is a schematic perspective view illustrating a rechargeablelithium battery according to an embodiment.

FIG. 2 is a scanning electron microscopic photograph of the secondpositive active material of Example 1.

FIG. 3 is a scanning electron microscopic photograph of the secondpositive active material of Comparative Example 1.

FIG. 4 is a scanning electron microscopic photograph of the secondpositive active material of Comparative Example 2.

DETAILED DESCRIPTION

Hereinafter, example embodiments will be described in more detail sothat those of ordinary skill in the art can easily implement them.However, the subject matter of this disclosure may be embodied in manydifferent forms and should not be construed as being limited to theexample embodiments set forth herein.

The terminology used herein is used to describe embodiments only, and isnot intended to limit the present disclosure. The singular expressionincludes the plural expression unless the context clearly dictatesotherwise.

As used herein, the term “a combination thereof” refers to a mixture, alaminate, a composite, a copolymer, an alloy, a blend, a reactionproduct, and/or the like of constituents.

Herein, it should be understood that terms such as “comprises,”“includes,” or “have” are intended to designate the presence of anembodied feature, number, step, element, or a combination thereof, butit does not preclude the possibility of the presence or addition of oneor more other features, number, step, element, or a combination thereof.

In the drawings, the thickness of layers, films, panels, regions, etc.,may be exaggerated for clarity and like reference numerals designatelike elements throughout the specification. It will be understood thatwhen an element such as a layer, film, region, or substrate is referredto as being “on” another element, it can be directly on the otherelement or intervening elements may also be present. In contrast, whenan element is referred to as being “directly on” another element, thereare no intervening elements present.

In addition, the term “layer,” as used herein, includes not only a shapeformed on the whole surface when viewed from a plan view, but also ashape formed on a partial surface.

In addition, the average particle diameter may be measured by anysuitable method generally used in the art. For example, the averageparticle diameter may be measured by a particle size analyzer, or may bemeasured by a transmission electron micrograph (TEM) or a scanningelectron micrograph (SEM). In some embodiments, it is possible to obtainan average particle diameter value by measuring using a dynamic lightscattering method, performing data analysis, counting the number ofparticles for each particle size range, and calculating the averageparticle diameter from the results. Unless otherwise defined, theaverage particle diameter is measured by a particle size analyzer andmay mean the diameter (D50) of particles having a cumulative volume of50 volume % in the particle size distribution.

Positive Active Material

In an embodiment, a positive active material for a rechargeable lithiumbattery includes a first positive active material in a form of secondaryparticles formed by aggregation of a plurality of primary particles, anda second positive active material having a single crystal form, whereinboth of the first positive active material and the second positiveactive material are nickel-based positive active materials and arecoated with cobalt, respectively, and a maximum roughness of the surfaceof the second positive active material is greater than or equal to about15 nm. Such a positive active material may exhibit improved cycle-lifecharacteristics while implementing high capacity and high energydensity.

First Positive Active Material

Embodiments of the first positive active material have a polycrystalform, and include secondary particles formed by aggregation of at leasttwo or more primary particles.

The first positive active material according to an embodiment is coatedwith cobalt. For example, the secondary particles of the first positiveactive material may be coated with cobalt on the surface of thesecondary particles. In some embodiments, the first positive activematerial may include the secondary particles and cobalt-coating layerson the surfaces of the secondary particles. Embodiments of the firstpositive active material are coated with cobalt, and thus, structuralcollapse of the first positive active material resulting from repetitivecharge and discharge cycles is effectively suppressed or reduced, andaccordingly, room temperature and high temperature cycle-lifecharacteristics may be improved.

Herein, the cobalt coating may be expressed or formed by coating acobalt-containing compound. The cobalt-containing compound may, forexample, include cobalt oxide, cobalt sulfate salt, cobalt nitrate salt,cobalt hydroxide, cobalt carbonate, a compound thereof, a mixturethereof, and/or the like, which may further include lithium, nickel,and/or the like.

The amount of cobalt coating in the first positive active material maybe about 0.01 mol % to about 7 mol %, for example, about 0.01 mol % toabout 6 mol %, about 0.05 mol % to about 5 mol %, about 0.1 mol % toabout 4 mol %, about 0.1 mol % to about 3 mol %, or about 0.5 mol % toabout 3 mol %, and may also be about 0.01 atom % to about 7 atom %,about 0.1 atom % to about 5 atom %, or about 0.5 atom % to about 3 atom% based on the total amount of the first positive active material.Embodiments of the rechargeable lithium battery including the firstpositive active material may implement excellent room temperature andhigh temperature cycle-life characteristics.

The thickness of the cobalt coating layer in the first positive activematerial may vary depending on the firing temperature during coating,and cobalt may penetrate into the active material and be coated onand/or doped into the first positive active material according to thefiring temperature. Accordingly, the thickness of the cobalt coatinglayer may be, for example, about 1 nm to about 2 μm, about 1 nm to about1.5 pm, about 1 nm to about 1 μm, about 1 nm to about 900 nm, about 1 nmto about 700 nm, about 1 nm to about 500 nm, about 1 nm to about 300 nm,about 5 nm to about 100 nm, or about 5 nm to about 50 nm. Embodiments ofthe rechargeable lithium battery including the first positive activematerial may exhibit excellent room temperature and high temperaturecycle-life characteristics.

The particle diameter of the first positive active material, forexample, the average particle diameter of the secondary particles may beabout 7 μm to about 25 μm. For example, the particle diameter of thefirst positive active material (or the average particle diameter of thesecondary particles) may be about 9 μm to about 25 μm, about 12 μm toabout 25 μm, about 15 μm to about 25 μm, or about 10 μm to about 20 μm.The average particle diameter of the secondary particles of the firstpositive active material may be equal to or larger than the averageparticle diameter of the second positive active material having a singlecrystal form, which will be further described herein below. The positiveactive material according to an embodiment may be in the form of amixture of a first positive active material, which has polycrystallineform and is in the form of large particles, and a second positive activematerial, which has a single crystal form and is in the form of smallparticles, thereby improving a mixture density, and providing highcapacity and high energy density.

The first positive active material may include a lithium nickelcomposite oxide (or a first nickel-based oxide) as a nickel-basedpositive active material. The nickel content in the lithium nickelcomposite oxide may be greater than or equal to about 30 mol %, forexample greater than or equal to about 40 mol %, greater than or equalto about 50 mol %, greater than or equal to about 60 mol %, greater thanor equal to about 70 mol %, greater than or equal to about 80 mol %, orgreater than or equal to about 90 mol % and less than or equal to about100 mol %, less than or equal to about 99.9 mol % or less than or equalto about 99 mol %, or any range subsumed therein, based on the totalamount of elements excluding lithium and oxygen. For example, the nickelcontent in the lithium nickel composite oxide may be higher than thecontent of each of other metals such as, for example, cobalt, manganese,and aluminum. When the nickel content satisfies the above range, thepositive active material may exhibit excellent battery performance whilerealizing a high capacity.

In some embodiments, the first positive active material may include acompound represented by Chemical Formula 1.

Li_(a1)Ni_(x1)M¹ _(y1)M² _(1−x1−y1)O₂   Chemical Formula 1

In Chemical Formula 1, 0.9≤a1≤1.8, 0.3≤x1≤1, 0≤y1≤0.7, and M¹ and M² areeach independently selected from Al, B, Ba, Ca, Ce, Co, Cr, Cu, F, Fe,Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr, and a combination thereof.

The first positive active material may include, for example, a compoundof Chemical Formula 2.

Li_(a2)Ni_(x2)Co_(y2)M³ _(1−x2−y2)O₂   Chemical Formula 2

In Chemical Formula 2, 0.9≤a2≤1.8, 0.3≤x2≤1, 0<y2≤0.7, and M³ isselected from Al, B, Ba, Ca, Ce, Cr, Cu, F, Fe, Mg, Mn, Mo, Nb, P, S,Si, Sr, Ti, V, W, Zr, and a combination thereof.

The first positive active material may include, for example, a compoundof Chemical Formula 3.

Li_(a3)Ni_(x3)Co_(y3)Al_(z3)M⁴ _(1−x3−y3−z3)O₂   Chemical Formula 3

In Chemical Formula 3, 0.9≤a3≤1.8, 0.3≤x3<1, 0<y3<0.7, 0<z3<0.4, and M⁴is selected from B, Ba, Ca, Ce, Cr, Cu, F, Fe, Mg, Mn, Mo, Nb, P, S, Si,Sr, Ti, V, W, Zr, and a combination thereof.

In the positive active material according to an embodiment, the firstpositive active material may be included in an amount of about 50 wt %to about 90 wt %, and the second positive active material may beincluded in an amount of about 10 wt % to about 50 wt % based on thetotal amount of the first positive active material and the secondpositive active material. The first positive active material may be forexample included in an amount of about 60 wt % to about 90 wt %, orabout 70 wt % to about 90 wt % and the second positive active materialmay be for example included in an amount of about 10 wt % to about 40 wt%, or about 10 wt % to about 30 wt %. When the content ratio of thefirst positive active material and the second positive active materialis as described above, the positive active material including the samemay realize high capacity, improve a mixture density, and exhibit highenergy density.

Meanwhile, a maximum roughness (R_(max); peak to peak height) of thesurface of the first positive active material may be for example about 3nm to about 100 nm, or about 5 nm to about 50 nm. An average roughness(R_(a)) of the surface of the first positive active material may beabout 0.2 nm to about 10 nm, or about 0.5 nm to about 3 nm. In addition,a root mean square roughness (R_(q)) of the surface of the firstpositive active material may be about 0.5 nm to about 10 nm, or about0.7 nm to about 3 nm. Details such as the meaning and measurement methodof the maximum roughness, the average roughness, and the root meansquare roughness will be further described herein below in the sectionon the second positive electrode active material.

Second Positive Active Material

The second positive active material is in a single crystal form, whereinthe single crystal form means that one single particle is present alonewithout a grain boundary thereinside, and is a monolithic structure inwhich particles are not aggregated together with one another but presentas an independent phase in terms of morphology, and thus may beexpressed as a single crystal particle. The positive active materialaccording to the embodiment may exhibit improved cycle-lifecharacteristics while implementing high capacity and high energy densityby including the second positive active material.

The second positive active material has no particular limit to a shapebut may have various suitable shapes such as a polyhedron, an oval, aplate, a rod, an irregular shape, and/or the like.

The second positive active material according to an embodiment is coatedwith cobalt. For example, the surface of the second positive activematerial may be coated with the cobalt-containing compound. The secondpositive active material may include a single crystal and a cobaltcoating layer on the surfaces of the single crystal. Because the secondpositive active material is coated with cobalt, structural collapse ofthe second positive active material from repeated charges and dischargesis effectively suppressed or reduced, and thus, room temperature andhigh temperature cycle-life characteristics may be improved.

A method of preparing the positive active material according to anembodiment, which is further described herein below, may be performednot by separately coating the first positive active material and thesecond positive active material but by coating the mixture together bymixing them and then, concurrently (e.g., simultaneously) performingcoating and firing through a third heat-treatment. Accordingly, thesecond positive active material of the cobalt-coated single crystals hasnot a smooth or flat surface but an uneven surface having set orspecific protrusions and depressions. Accordingly, the surface roughnessof the second positive active material is increased, and a specificsurface area thereof is also increased. The second positive activematerial according to an embodiment may improve charge and dischargeefficiency and cycle-life characteristics of a battery due to theincreased surface roughness and specific surface area, compared with anexisting single crystal positive active material coated with cobaltand/or the like.

The second positive active material according to an embodiment hasprotrusions and depressions on the surface, for example, linearprotrusions and depressions or non-linear protrusions and depressions.For example, the cobalt-containing compound may be attached to thesurface of the second positive active material of single crystals, forexample, linearly or atypically attached thereto, to thereby cover thesurface of the single crystal in an uneven form. This coating shape isdistinct from an existing island-type coating (e.g., a coating havingdiscrete and non-contiguous islands).

This second positive active material exhibits high surface roughness.The surface roughness may be measured by using an image taken withatomic force microscope (AFM) and/or the like, for example, an opticalprofiler. Maximum roughness (R_(max); peak to peak height; maximumroughness depth) may be a vertical distance between the highest peak andthe lowest valley within a reference length of a roughness cross-sectioncurve (roughness profile). Average roughness (R_(a)) may also bereferred to as center line average roughness, which is obtained as anarithmetic average of absolute values of ordinates (length from centerto peak) within the reference length of the roughness profile. Root meansquare roughness (R_(q)) may be a root average square (rms) of theordinates within the reference length of the roughness profile. As forsuch surface roughness, parameters and measurement methods defined in KSB 0601 or ISO 4287/1 may be referenced.

The maximum roughness (R_(max); peak to peak height) of the surface ofthe second positive active material may be greater than or equal toabout 15 nm, for example, greater than or equal to about 20 nm, or maybe about 15 nm to about 100 nm, about 15 nm to about 50 nm, about 15 nmto about 40 nm, or about 20 nm to about 35 nm. In this case, thepositive active material for a rechargeable lithium battery includingthe second positive active material exhibits high energy density andhigh capacity, and may implement excellent charge/discharge efficiencyand cycle-life characteristics.

An average roughness (R_(a)) of the surface of the second positiveactive material may be greater than or equal to about 1.5 nm, forexample, greater than or equal to about 1.8 nm, about 1.5 nm to about 10nm, about 1.5 nm to about 8.0 nm, about 1.5 nm to about 6.0 nm, about1.8 nm to about 5.0 nm, about 2.0 nm to about 10 nm, or about 3.0 nm toabout 10 nm. In this case, the positive active material for arechargeable lithium battery including the second positive activematerial may exhibit high energy density and high capacity, and mayimplement excellent charge/discharge efficiency and cycle-lifecharacteristics.

A root mean square roughness (R_(q)) of the surface of the secondpositive active material may be greater than or equal to about 2.0 nm,for example, greater than or equal to about 2.3 nm, and may be about 2.0nm to about 10 nm, about 2.0 nm to about 8 nm, about 2.0 nm to about 6nm, about 2.3 nm to about 5 nm, about 3.0 nm to about 10 nm, or about4.0 nm to about 10 nm. In this case, the positive active material for arechargeable lithium battery including the second positive activematerial exhibits high energy density and high capacity, and mayimplement excellent charge/discharge efficiency and cycle-lifecharacteristics.

The BET specific surface area of the entire positive active materialincluding the first positive active material and the second positiveactive material may be about 0.3 m²/g to about 0.6 m²/g, for example,about 0.3 m²/g to about 0.5 m²/g, or about 0.3 m²/g to about 0.4 m²/g.In this case, the positive active material may realize excellentcharge/discharge efficiency and cycle-life characteristics.

The cobalt amount in the second positive active material may be about0.01 mol % to about 7 mol %, for example, about 0.01 mol % to about 6mol %, about 0.05 mol % to about 5 mol %, about 0.1 mol % to about 4 mol%, about 0.1 mol % to about 3 mol %, or about 0.5 mol % to about 3 mol %and may be also be about 0.01 atom % to about 7 atom %, about 0.1 atom %to about 5 atom %, or about 0.5 atom % to about 3 atom % based on thetotal amount of the second positive active material. In this case, therechargeable lithium battery including the second positive activematerial may implement excellent room temperature and high temperaturecycle-life characteristics.

The thickness of the cobalt coating layer in the second positive activematerial may be about 1 nm to about 2 μm, for example, about 1 nm toabout 1 μm, about 1 nm to about 900 nm, about 1 nm to about 700 nm,about 1 nm to about 500 nm, about 1 nm to about 300 nm, about 5 nm toabout 100 nm, or about 5 nm to about 50 nm. In this case, therechargeable lithium battery including the second positive activematerial may exhibit excellent room temperature and high temperaturecycle-life characteristics.

The average particle diameter of the second positive active material,for example, the average particle diameter of the single crystal may beabout 1 μm to about 10 μm, for example, about 1 μm to about 8 μm, about2 μm to about 7 μm, about 2 μm to about 6 μm, about 2 μm to about 5 μm,and, for example, may be about 2 μm to about 4 μm. The average particlediameter of the second positive active material may be the same as orsmaller than that of the first positive active material, and thus thedensity of the positive active material may be further increased.

The second positive active material may include a lithium nickel-basedcomposite oxide (or a second nickel-based oxide) as a nickel-basedactive material. The nickel content in the lithium nickel compositeoxide may be greater than or equal to about 30 mol %, for examplegreater than or equal to about 40 mol %, greater than or equal to about50 mol %, greater than or equal to about 60 mol %, greater than or equalto about 70 mol %, greater than or equal to about 80 mol %, or greaterthan or equal to about 90 mol % and less than or equal to about 100 mol%, less than or equal to about 99.9 mol %, or less than or equal toabout 99 mol % based on the total amount of elements excluding lithiumand oxygen. For example, the nickel content in the lithium nickelcomposite oxide may be higher than the content of each of the othertransition metals such as cobalt, manganese, and aluminum. When thenickel content satisfies the above range, the positive active materialmay exhibit excellent battery performance while realizing a highcapacity.

The second positive active material may include for example a compoundrepresented by Chemical Formula 11.

Li_(a11)Ni_(x11)M¹¹ _(y11)M¹² _(1−x11−y11)O₂   Chemical Formula 11

In Chemical Formula 11, 0.9≤a11≤1.8, 0.3≤x11≤1, 0≤y11≤0.7, and M¹¹ andM¹² are each independently selected from Al, B, Ba, Ca, Ce, Co, Cr, Cu,F, Fe, Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr, and a combinationthereof.

In Chemical Formula 11, x11 representing the nickel content may be, forexample, 0.4≤x11<1, 0.5≤x11<1, 0.6≤x11<1, 0.8≤x11<1, or 0.9≤x11<1.Herein, the positive active material including the same may implement ahigh capacity.

The second positive active material may be, for example, represented byChemical Formula 12 or Chemical Formula 13.

Li_(a12)Ni_(x12)Co_(y12)Mn_(z12)M¹³ _(1−x12−y12−z12)O₂   ChemicalFormula 12

In Chemical Formula 12, 0.9≤a12≤1.8, 0.3≤x12<1, 0<y12<0.7, 0<z12<0.7,and M¹³ is selected from Al, B, Ba, Ca, Ce, Cr, Cu, F, Fe, Mg, Mo, Nb,P, S, Si, Sr, Ti, V, W, Zr, and a combination thereof.

Li_(a13)Ni_(x13)Co_(y13)M¹⁴ _(1−x13−y13)O₂   Chemical Formula 13

In Chemical Formula 13, 0.9≤a13≤1.8, 0.3≤x13<1, 0<y13≤0.7, and M¹⁴ isselected from Al, B, Ba, Ca, Ce, Cr, Cu, F, Fe, Mg, Mn, Mo, Nb, P, S,Si, Sr, Ti, V, W, Zr, and a combination thereof.

When the second positive active material includes the compoundrepresented by Chemical Formula 12 or Chemical Formula 13, the initialdischarge capacity is not lowered while implementing a high capacity,and the effect of improving cycle-life characteristics can be obtained.

In Chemical Formula 12, x12, y12, and z12 may be, for example, in therange: 0.5≤x12<1, 0<y12<0.5, and 0<z12<0.5, 0.6≤x12<1, 0<y12<0.4, and0<z12<0.4, or 0.8≤x12<1, 0<y12<0.2, and 0<z12<0.2. In Chemical Formula13, x13 and y13 may be, for example, in the range: 0.5≤x13<1 and0<y13≤0.5, 0.6≤x13<1 and 0<y13≤0.4, or 0.8≤x13≤0.99, and 0.01≤y13≤0.2.

Method of Preparing Positive Active Material

In an embodiment, a method of preparing a positive active material for arechargeable lithium battery includes mixing a first nickel-basedhydroxide and a lithium raw material and performing a firstheat-treatment to prepare a first nickel-based oxide, mixing a secondnickel-based hydroxide and a lithium raw material and performing asecond heat-treatment to prepare a second nickel-based oxide, and mixingthe first nickel-based oxide, the second nickel-based oxide, and acobalt compound and performing a third heat-treatment to perform acobalt coating, obtaining a final positive active material including thefirst positive active material and the second positive active material.

Herein, the first nickel-based oxide and first positive active materialhave a form of secondary particles formed by aggregation of a pluralityof primary particles and the second nickel-based oxide and secondpositive active material have a single crystal form. The first positiveactive material may be a material that the first nickel-based oxide iscoated with cobalt on the surface, and the second positive activematerial may be a material that the second nickel-based oxide is coatedwith cobalt on the surface.

In an embodiment, the first positive active material and the secondpositive active material may be prepared by not individually coating thefirst nickel-based oxide and the second nickel-based oxide butconcurrently (e.g., simultaneously) coating them after first mixing themtogether. Accordingly, because the surface of the cobalt-coated singlecrystal second positive active material is not smooth and flat butuneven, the cobalt-coated single crystal second positive active materialhas a high surface roughness and a high specific surface area.Accordingly, a positive active material for a rechargeable lithiumbattery including this second positive active material may exhibit ahigh specific surface area and thus realize excellent capacitycharacteristics and cycle-life characteristics.

The first nickel-based hydroxide and the second nickel-based hydroxideare precursors of the positive active material and may be eachindependently be a nickel hydroxide, a nickel-based composite hydroxidecontaining an element other than nickel, or a nickel-transition metalelements composite hydroxide containing a transition metal other thannickel.

For example, the first nickel-based hydroxide and second nickel-basedhydroxide may each independently be represented by Chemical Formula 21.

Ni_(x21)M²¹ _(y21)M²² _(1−x21−y21)(OH)₂

In Chemical Formula 21, 0.3≤x21≤1, 0≤y21≤0.7, and M²¹ and M²² are eachindependently selected from Al, B, Ba, Ca, Ce, Co, Cr, Cu, F, Fe, Mg,Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr, and a combination thereof.

The first nickel-based hydroxide may have a particle diameter of about 7μm to about 25 μm, for example, about 10 μm to about 25 μm, about 15 μmto about 25 μm, or about 10 μm to about 20 μm. The second nickel-basedhydroxide may have a particle diameter of about 1 μm to about 10 μm, forexample, about 2 μm to about 9 μm, about 2 μm to about 8 μm, or about 3μm to about 7 μm.

The lithium raw material is a lithium source of the positive activematerial and may include, for example, Li₂CO₃, LiOH, a hydrate thereof,or a combination thereof.

When the first nickel-based hydroxide is mixed with the lithium rawmaterial, a ratio of a mole number of lithium in the lithium rawmaterial relative to a mole number of elements excluding H and Oincluded in the first nickel-based hydroxide, for example, may begreater than or equal to about 0.8, greater than or equal to about 0.85,greater than or equal to about 0.9, greater than or equal to about 0.95,or greater than or equal to about 1.0 and less than or equal to about1.8, less than or equal to about 1.5, less than or equal to about 1.2,less than or equal to about 1.1, or less than or equal to about 1.05.

The first heat-treatment may be performed under an oxidizing gasatmosphere, for example, under an oxygen atmosphere or an airatmosphere. In addition, the first heat-treatment may be performed atabout 600° C. to about 900° C. or about 600° C. to about 800° C., forexample, for about 5 hours to about 20 hours, or, for example, 5 hoursto 15 hours. The first nickel-based oxide obtained through the firstheat-treatment may be referred to as a first lithium nickel-based oxide.

When the second nickel-based hydroxide is mixed together with thelithium raw material, a ratio of a mole number of lithium in the lithiumraw material relative to a mole number of elements excluding H and 0included in the second nickel-based hydroxide may be, for example,greater than or equal to about 0.8, greater than or equal to about 0.85,greater than or equal to about 0.9, greater than or equal to about 0.95,or greater than or equal to about 1.0 and less than or equal to about1.8, less than or equal to about 1.5, less than or equal to about 1.2,less than or equal to about 1.1, or less than or equal to about 1.05.

The second heat-treatment also may be performed under the oxidizing gasatmosphere, for example, under the oxygen atmosphere or under the airatmosphere. In addition, the second heat-treatment may be performed, forexample, at about 800° C. to about 1100° C., or about 900° C. to about1000° C., for example, for about 5 hours to about 20 hours, or about 5hours to about 15 hours. The second nickel-based oxide obtained throughthe second heat-treatment may be referred to as a second lithiumnickel-based oxide.

The second nickel-based oxide has a single crystal form, which may beobtained through adjustment of conditions such as a temperature, time,and the like of the second heat-treatment or through various suitableconditions during synthesis of the second nickel-based hydroxide in aco-precipitation method.

The method of preparing a positive active material for a rechargeablelithium battery may further include pulverizing a product obtained aftermixing and secondarily heat-treating the second nickel-based hydroxideand the lithium raw material, thereby obtaining the single crystalsecond nickel-based oxide. The pulverization may be performed by usingvarious suitable pulverizing devices such as a jet mill and/or the like.Herein, the pulverization of the obtained product is a process ofobtaining a single-crystal active material, which is distinguished fromcrushing of a general active material.

When the first nickel-based oxide is mixed with the second nickel-basedoxide, the first nickel-based oxide and the second nickel-based oxidemay have a weight ratio of about 9:1 to about 5:5, for example, about9:1 to about 6:4, or about 8:2 to about 7:3. When the first nickel-basedoxide and the second nickel-based oxide are mixed together within theaforementioned range, the obtained positive active material may exhibithigh-capacity, high energy density and high electrode plate density.

The mixture of the first nickel-based oxide and the second nickel-basedoxide is cobalt-coated. The cobalt coating may be performed in a dry orwet method.

For example, the dry coating may be performed by thirdly heat-treatingthe mixture after adding a cobalt compound to the mixture. While thecobalt-containing compound is added to the mixture, a lithium-containingcompound such as lithium hydroxide may be added together. Thelithium-containing compound may be added in an amount of about 0.01parts by mole to about 5 parts by mole, or about 0.1 parts by mole toabout 3 parts by mole of the total amount of elements excluding lithiumand oxygen in the total positive active material.

Or, while the mixture is being mixed and washed with distilled water andthe like by adding the mixture thereto, the cobalt compound is addedthereto to perform wet coating, and then, the third heat-treatment maybe performed. While the cobalt-containing compound is added, alithium-containing compound such as lithium hydroxide and/or a pHadjusting agent such as sodium hydroxide may be added together.

The cobalt compound may be mixed to include cobalt in an amount of about0.01 parts by mole to about 7 parts by mole, about 0.01 parts by mole toabout 5 parts by mole, or about 0.1 parts by mole to about 3 parts bymole, based on 100 parts by mole of the total amount of elementsexcluding lithium and oxygen in the total positive active material. Thecobalt-containing compound may, for example, include cobalt hydroxide,cobalt carbonate, cobalt sulfate salt, cobalt oxide, cobalt nitratesalt, a compound thereof, a mixture thereof, or the like, which mayfurther include lithium, nickel, and/or the like.

The third heat-treatment may be performed under an oxidizing gasatmosphere. The oxidizing gas atmosphere may be an oxygen or airatmosphere. The third heat-treatment may be performed, for example,about 650° C. to about 900° C. or about 650° C. to about 800° C. Thethird heat-treatment may be performed during variable time depending ona heat-treatment temperature and/or the like, for example, for about 5hours to about 30 hours or about 10 hours to about 24 hours.

Subsequently, when the heat-treatment is completed, the heat-treatedproduct is cooled down to room temperature, obtaining the aforementionedpositive active material for a rechargeable lithium battery according toan embodiment. The prepared positive active material is in a state thatthe first positive active material including secondary particles whichare formed of the aggregated primary particles is mixed together withthe second positive active material having a single crystal form,wherein the first and second positive active materials are respectivelycoated with cobalt, and protrusions and depressions are formed on thesurface of the second positive active material.

Positive Electrode

A positive electrode for a rechargeable lithium battery may include acurrent collector and a positive active material layer on the currentcollector. The positive active material layer may include a positiveactive material, and may further include a binder and/or a conductivematerial.

The binder improves binding properties of positive active materialparticles together with one another and together with a currentcollector. Examples thereof may be polyvinyl alcohol, carboxylmethylcellulose, hydroxypropyl cellulose, diacetyl cellulose,polyvinylchloride, carboxylated polyvinylchloride, polyvinylfluoride, anethylene oxide-containing polymer, polyvinylpyrrolidone, polyurethane,polytetrafluoroethylene, polyvinylidene fluoride, polyethylene,polypropylene, a styrene-butadiene rubber, an acrylatedstyrene-butadiene rubber, an epoxy resin, nylon, and the like, but arenot limited thereto.

The content of the binder in the positive active material layer may beabout 1 wt % to about 5 wt % based on the total weight of the positiveactive material layer.

The conductive material is included to provide electrode conductivity(e.g., electrical conductivity). Any suitable electrically conductivematerial may be used as a conductive material unless it causes achemical change (e.g., unless it causes an undesirable or unsuitablechemical change in the resultant battery). Examples of the conductivematerial may include a carbon-based material such as natural graphite,artificial graphite, carbon black, acetylene black, ketjen black, acarbon fiber, carbon nanotube, and the like; a metal-based material of ametal powder or a metal fiber including copper, nickel, aluminum,silver, and the like; a conductive polymer such as a polyphenylenederivative; or a mixture thereof.

The content of the conductive material in the positive active materiallayer may be about 1 wt % to about 5 wt % based on the total weight ofthe positive active material layer.

An aluminum foil may be used as the current collector, but is notlimited thereto.

Negative Electrode

A negative electrode for a rechargeable lithium battery includes acurrent collector and a negative active material layer on the currentcollector. The negative active material layer may include a negativeactive material, and may further include a binder and/or a conductivematerial.

The negative active material may include a material that reversiblyintercalates/deintercalates lithium ions, a lithium metal, a lithiummetal alloy, a material capable of doping/dedoping lithium, ortransition metal oxide.

The material that reversibly intercalates/deintercalates lithium ionsmay include, for example crystalline carbon, amorphous carbon, or acombination thereof as a carbon-based negative active material. Thecrystalline carbon may be non-shaped, or sheet, flake, spherical, orfiber shaped natural graphite and/or artificial graphite.

The amorphous carbon may be a soft carbon, a hard carbon, a mesophasepitch carbonization product, calcined coke, and/or the like.

The lithium metal alloy includes an alloy of lithium and a metalselected from Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba,Ra, Ge, Al, and Sn.

The material capable of doping/dedoping lithium may be a Si-basednegative active material and/or a Sn-based negative active material. TheSi-based negative active material may include silicon, a silicon-carboncomposite, SiO_(x) (0<x<2), and/or a Si-Q alloy (wherein Q is an alkalimetal, an alkaline-earth metal, a Group 13 element, a Group 14 element,a Group 15 element, a Group 16 element, a transition metal, a rare earthelement, and a combination thereof, but not Si) and the Sn-basednegative active material may include Sn, SnO₂, and/or Sn—R alloy(wherein R is an alkali metal, an alkaline-earth metal, a Group 13element, a Group 14 element, a Group 15 element, a Group 16 element, atransition metal, a rare earth element, and a combination thereof, butnot Sn). At least one of these materials may be mixed together withSiO₂. The elements Q and R may be selected from Mg, Ca, Sr, Ba, Ra, Sc,Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru,Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, TI, Ge,P, As, Sb, Bi, S, Se, Te, Po, and a combination thereof.

The silicon-carbon composite may be, for example, a silicon-carboncomposite including a core including crystalline carbon and siliconparticles and an amorphous carbon coating layer on the surface of thecore. The crystalline carbon may be artificial graphite, naturalgraphite, or a combination thereof. The amorphous carbon precursor maybe a coal-based pitch, mesophase pitch, petroleum-based pitch,coal-based oil, petroleum-based heavy oil, and/or a polymer resin suchas a phenol resin, a furan resin, and/or a polyimide resin. In thiscase, the content of silicon may be about 10 wt % to about 50 wt % basedon the total weight of the silicon-carbon composite. In addition, thecontent of the crystalline carbon may be about 10 wt % to about 70 wt %based on the total weight of the silicon-carbon composite, and thecontent of the amorphous carbon may be about 20 wt % to about 40 wt %based on the total weight of the silicon-carbon composite. In addition,a thickness of the amorphous carbon coating layer may be about 5 nm toabout 100 nm. An average particle diameter (D50) of the siliconparticles may be about 10 nm to about 20 μm.

The average particle diameter (D50) of the silicon particles may be, forexample, about 10 nm to about 200 nm. The silicon particles may exist inan oxidized form, and in this case, an atomic content ratio of Si:O inthe silicon particles indicating a degree of oxidation may be about 99:1to about 33:67. The silicon particles may be SiO_(x) particles, and inthis case, the range of x in SiO_(x) may be greater than about 0 andless than about 2. In the present specification, unless otherwisedefined, an average particle diameter (D50) indicates a diameter of aparticle where an accumulated volume is about 50 volume % in a particlesize distribution.

The Si-based negative active material or Sn-based negative activematerial may be mixed together with the carbon-based negative activematerial. When the Si-based negative active material or Sn-basednegative active material and the carbon-based negative active materialare mixed together and used, the mixing ratio may be a weight ratio ofabout 1:99 to about 90:10.

In the negative active material layer, the negative active material maybe included in an amount of about 95 wt % to about 99 wt % based on thetotal weight of the negative active material layer.

In an embodiment, the negative active material layer further includes abinder, and may optionally further include a conductive material (anelectrically conductive material). The content of the binder in thenegative active material layer may be about 1 wt % to about 5 wt % basedon the total weight of the negative active material layer. In addition,when the conductive material is further included, the negative activematerial layer may include about 90 wt % to about 98 wt % of thenegative active material, about 1 wt % to about 5 wt % of the binder,and about 1 wt % to about 5 wt % of the conductive material.

The binder serves to well adhere the negative active material particlesto each other and also to adhere the negative active material to thecurrent collector. The binder may be a water-insoluble binder, awater-soluble binder, or a combination thereof.

Examples of the water-insoluble binder include polyvinyl chloride,carboxylated polyvinyl chloride, polyvinyl fluoride, an ethyleneoxide-containing polymer, an ethylene propylene copolymer, polystyrene,polyvinylpyrrolidone, polyurethane, polytetrafluoro ethylene,polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide,polyimide, or a combination thereof.

The water-soluble binder may include a rubber binder and/or a polymerresin binder. The rubber binder may be selected from a styrene-butadienerubber, an acrylated styrene-butadiene rubber, anacrylonitrile-butadiene rubber, an acrylic rubber, a butyl rubber, afluororubber, and a combination thereof. The polymer resin binder may beselected from polyethylene oxide, polyvinylpyrrolidone,polyepichlorohydrin, polyphosphazene, polyacrylonitrile, an ethylenepropylene diene copolymer, polyvinylpyridine, chlorosulfonatedpolyethylene, latex, a polyester resin, an acrylic resin, a phenolresin, an epoxy resin, polyvinyl alcohol, and a combination thereof.

When a water-soluble binder is used as the negative electrode binder, acellulose-based compound capable of imparting viscosity may be furtherincluded as a thickener. As the cellulose-based compound, one or more ofcarboxymethyl cellulose, hydroxypropylmethyl cellulose, methylcellulose, or alkali metal salts thereof may be mixed together and used.As the alkali metal, Na, K, and/or Li may be used. The amount of thethickener used may be about 0.1 parts by weight to about 3 parts byweight based on 100 parts by weight of the negative active material.

The conductive material is included to provide electrode conductivity.Any suitable electrically conductive material may be used as aconductive material unless it causes a chemical change (e.g., unless itcauses an undesirable or unsuitable chemical change in the resultantbattery). Examples of the conductive material include a carbon-basedmaterial such as natural graphite, artificial graphite, carbon black,acetylene black, ketjen black, a carbon fiber, carbon nanotube, and thelike; a metal-based material of a metal powder or a metal fiberincluding copper, nickel, aluminum, silver, and the like; a conductivepolymer such as a polyphenylene derivative; or a mixture thereof.

The current collector may include one selected from a copper foil, anickel foil, a stainless steel foil, a titanium foil, a nickel foam, acopper foam, a polymer substrate coated with a conductive metal, and acombination thereof.

Rechargeable Lithium Battery

Another embodiment provides a rechargeable lithium battery including apositive electrode, a negative electrode, a separator between thepositive electrode and the positive electrode, and an electrolyte. Here,the aforementioned electrode may be the positive electrode and negativeelectrode.

FIG. 1 is a schematic perspective view illustrating a rechargeablelithium battery according to an embodiment. Referring to FIG. 1 , arechargeable lithium battery 100 according to an embodiment includes abattery cell including a positive electrode 114, a negative electrode112 facing the positive electrode 114, a separator 113 between thepositive electrode 114 and the negative electrode 112, and anelectrolyte for a rechargeable lithium battery impregnating the positiveelectrode 114, the negative electrode 112, and the separator 113, abattery case 120 housing the battery cell, and a sealing member 140sealing the battery case 120.

The electrolyte includes a non-aqueous organic solvent and a lithiumsalt.

The non-aqueous organic solvent serves as a medium for transmitting ionstaking part in the electrochemical reaction of a battery. Thenon-aqueous organic solvent may be a carbonate-based, ester-based,ether-based, ketone-based, alcohol-based solvent, or aprotic solvent.Examples of the carbonate-based solvent include dimethyl carbonate(DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropylcarbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate(MEC), ethylene carbonate (EC), propylene carbonate (PC), butylenecarbonate (BC), and the like. Examples of the ester-based solventinclude methyl acetate, ethyl acetate, n-propyl acetate, dimethylacetate, methyl propionate, ethyl propionate, γ-butyrolactone,decanolide, valerolactone, mevalonolactone, caprolactone, and the like.The ether-based solvent may be dibutyl ether, tetraglyme, diglyme,dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, and/or thelike and the ketone-based solvent may be cyclohexanone, and/or the like.In addition, the alcohol-based solvent may be ethyl alcohol, isopropylalcohol, etc. and the aprotic solvent may be nitriles such as R—CN(where R is a C2 to C20 linear, branched, or cyclic hydrocarbon groupand may include a double bond, an aromatic ring, or an ether bond),amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane,sulfolanes, and/or the like.

The non-aqueous organic solvent may be used alone or in a mixture. Whenthe organic solvent is used in a mixture, the mixture ratio may becontrolled in accordance with a desirable battery performance.

In addition, in the case of the carbonate-based solvent, a mixture of acyclic carbonate and a chain carbonate may be used. In this case, whenthe cyclic carbonate and the chain carbonate are mixed in a volume ratioof about 1:1 to about 1:9, the electrolyte may exhibit excellentperformance.

The non-aqueous organic solvent may further include an aromatichydrocarbon-based organic solvent in addition to the carbonate-basedsolvent. In this case, the carbonate-based solvent and the aromatichydrocarbon-based organic solvent may be mixed in a volume ratio ofabout 1:1 to about 30:1.

As the aromatic hydrocarbon-based organic solvent, an aromatichydrocarbon-based compound represented by Chemical Formula I may beused.

In Chemical Formula I, R⁴ to R⁹ are the same or different and areselected from hydrogen, a halogen, a C1 to C10 alkyl group, a C1 to C10haloalkyl group, and a combination thereof.

Examples of the aromatic hydrocarbon-based solvent may be selected frombenzene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene,1,4-difluorobenzene, 1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene,chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene,1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene,iodobenzene, 1,2-diiodobenzene, 1,3-diiodobenzene, 1,4-diiodobenzene,1,2,3-triiodobenzene, 1,2,4-triiodobenzene, toluene, fluorotoluene,2,3-difluorotoluene, 2,4-difluorotoluene, 2,5-difluorotoluene,2,3,4-trifluorotoluene, 2,3,5-trifluorotoluene, chlorotoluene,2,3-dichlorotoluene, 2,4-dichlorotoluene, 2,5-dichlorotoluene,2,3,4-trichlorotoluene, 2,3,5-trichlorotoluene, iodotoluene,2,3-diiodotoluene, 2,4-diiodotoluene, 2,5-diiodotoluene,2,3,4-triiodotoluene, 2,3,5-triiodotoluene, xylene, and a combinationthereof.

The electrolyte may further include vinylene carbonate or an ethylenecarbonate-based compound of Chemical Formula II as additive in order toimprove cycle-life of a battery.

In Chemical Formula II, R¹⁹ and R¹¹ are the same or different, and areselected from hydrogen, a halogen, a cyano group, a nitro group, andfluorinated C1 to C5 alkyl group, provided that at least one of R¹⁰ andR¹¹ is selected from a halogen, a cyano group, a nitro group, andfluorinated C1 to C5 alkyl group, but both of R¹⁰ and R¹¹ are nothydrogen.

Examples of the ethylene carbonate-based compound may be difluoroethylenecarbonate, chloroethylene carbonate, dichloroethylene carbonate,bromoethylene carbonate, dibromoethylene carbonate, nitroethylenecarbonate, cyanoethylene carbonate, or fluoroethylene carbonate. Theamount of the additive for improving cycle-life may be used within anappropriate or suitable range.

The lithium salt dissolved in the non-aqueous organic solvent supplieslithium ions in a battery, enables a basic operation of a rechargeablelithium battery, and improves transportation of the lithium ions betweenpositive and negative electrodes.

Examples of the lithium salt include at least one supporting saltselected from LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiN(SO₂C₂F₅)₂, Li(CF₃SO₂)₂N,LiN(SO₃C₂F₅)₂, Li(FSO₂)₂N (lithium bis(fluorosulfonyl)imide, LiC₄F₉SO₃,LiClO₄, LiAlO₂, LiAlCl₄, LiPO₂F₂, LiN(CxF_(2x+1)SO₂)(C_(y)F_(2y+1)SO₂),wherein x and y are natural numbers, for example, an integer in a rangefrom 1 to 20, lithium difluoro(bisoxolato) phosphate, LiCl, LiI,LiB(C₂O₄)₂ (lithium bis(oxalato) borate, LiBOB), and lithiumdifluoro(oxalato)borate (LiDFOB).

The lithium salt may be used in a concentration in a range from about0.1 M to about 2.0 M. When the lithium salt is included at the aboveconcentration range, an electrolyte may have excellent performance andlithium ion mobility due to optimal or suitable electrolyte conductivityand viscosity.

The separator 113 separates a positive electrode 114 and a negativeelectrode 112 and provides a transporting passage for lithium ions andmay be any suitable, generally-used separator in a lithium ion battery.In other words, the separator may have low resistance to ion transportand excellent impregnation for an electrolyte. For example, theseparator may include a glass fiber, polyester, polyethylene,polypropylene, polytetrafluoroethylene, or a combination thereof. It mayhave a form of a non-woven fabric or a woven fabric. For example, in alithium ion battery, a polyolefin-based polymer separator such aspolyethylene separator and polypropylene separator is mainly used. Inorder to ensure the heat resistance or mechanical strength, a coatedseparator including a ceramic component and/or a polymer material may beused. Optionally, it may have a mono-layered or multi-layered structure.

Rechargeable lithium batteries may be classified as lithium ionbatteries, lithium ion polymer batteries, and lithium polymer batteriesaccording to the presence of a separator and the kind of electrolyteused therein. The rechargeable lithium batteries may have a variety ofsuitable shapes and sizes, and include cylindrical, prismatic, coin, orpouch-type batteries (e.g., batteries having a pouch shape), and may bethin film batteries or may be rather bulky in size. Structures andmanufacturing methods for lithium ion batteries pertaining to thisdisclosure may be any suitable ones generally used in the art.

The rechargeable lithium battery according to an embodiment may be usedin an electric vehicle (EV), a hybrid electric vehicle such as a plug-inhybrid electric vehicle (PHEV), and portable electronic device becauseit implements a high capacity and has excellent storage stability,cycle-life characteristics, and high rate characteristics at hightemperatures.

Hereinafter, examples of embodiments of the present disclosure andcomparative examples are described. It is to be understood, however,that the examples are for the purpose of illustration and are not to beconstrued as limiting the present disclosure.

EXAMPLE 1 1. Preparation of First Nickel-Based Oxide in the Form ofSecondary Particles

First, a first nickel-based hydroxide (Ni_(0.95)Co_(0.04)Mn_(0.01)OH)prepared through a co-precipitation method is prepared. The firstnickel-based hydroxide and LiOH are mixed together, so that lithium ismixed in a mole ratio of 1.04 based on the total amount of elementsexcluding H and O of the first nickel-based hydroxide and then, theresultant is primarily heat-treated at about 750° C. for 15 hours underan oxygen atmosphere, obtaining a first nickel-based oxide(LiNi_(0.95)Co_(0.04)Mn_(0.01)O₂). The obtained first nickel-based oxidehas an average particle diameter of about 13.8 μm and is in the form ofsecondary particles composed of two or more primary particles.

2. Preparation of Second Nickel-Based Oxide in Single Crystal FormCo-Precipitation Process

Nickel sulfate (NiSO₄.6H₂O), cobalt sulfate (CoSO₄.7H₂O), and manganesesulfate (MnSO₄.H₂O) are dissolved in distilled water as a solvent,preparing a raw metal material mixed solution. In order to form acomplex, an ammonia water (NH₄OH) diluent and sodium hydroxide (NaOH) asa precipitant are prepared. Subsequently, the raw metal material mixedsolution, the ammonia water, and the sodium hydroxide are each put intoa reactor. While stirred, a reaction proceeds for about 20 hours.Subsequently, the resultant slurry solution in the reactor is filtered,washed with distilled water with high purity, and dried for 24 hours,obtaining a second nickel-based hydroxide(Ni_(0.94)Co_(0.05)Mn_(0.01)(OH)₂) powder. The obtained secondnickel-based hydroxide powder has an average particle diameter of about4.0 pm and a specific surface area of about 15 m²/g, which is measuredin a BET method.

Oxidation Process

The obtained second nickel-based hydroxide is mixed together with LiOHto satisfy Li/(Ni+Co+Mn)=1.05 and put in a furnace and then, secondarilyheat-treated at 910° C. for 8 hours under an oxygen atmosphere,obtaining a second nickel-based oxide (LiNi_(0.94)Co_(0.05)Mn_(0.01)O₂).Subsequently, the obtained second nickel-based oxide is pulverized forabout 30 minutes and then, separated/dispersed into a plurality ofsecond nickel-based oxide having a single crystal structure. The secondnickel-based oxide having a single crystal structure has an averageparticle diameter of about 3.7 μm.

3. Cobalt Coating and Preparation of Final Positive Active Material

The first nickel-based oxide and the second nickel-based oxide are mixedtogether in a weight ratio of 7:3, and the resultant mixture is washedin a weight ratio of 1:1 with water in a stirrer and dried at 150° C.Herein, 5 parts by mole of lithium hydroxide and 3 parts by mole ofcobalt oxide based on 100 parts by mole of transition metal elements ofthe nickel-based oxides are additionally mixed therewith and then, putin a furnace and thirdly heat-treated at about 700° C. for 15 hoursunder an oxygen atmosphere. Subsequently, the furnace is cooled down toroom temperature, obtaining a final positive active material in whichthe first positive active material and the second positive activematerial are mixed together.

The final positive active material is a mixture of the first positiveactive material in a secondary particle form and the second positiveactive material having a single crystal form, which are respectivelycoated with cobalt.

FIG. 2 is a scanning electron microscopic photograph of the secondpositive active material prepared according to Example 1. Referring toFIG. 2 , protrusions and depressions are formed on the surface of thesecond positive active material having a single crystal form.

4. Manufacture of Positive Electrode

95 wt % of the final positive active material, 3 wt % of apolyvinylidene fluoride binder, and 2 wt % of carbon nanotube conductivematerial are mixed together in an N-methylpyrrolidone solvent to preparepositive active material slurry. The resultant positive active materialslurry is applied to an aluminum current collector, dried, and thencompressed to manufacture a positive electrode.

5. Manufacture of Rechargeable Lithium Battery Cell

A coin half-cell is manufactured by providing a separator having apolyethylene polypropylene multilayer structure between the manufacturedpositive electrode and lithium metal negative electrode, and injectingan electrolyte solution in which 1.0 M LiPF₆ lithium salt was added to asolvent in which ethylene carbonate and diethyl carbonate are mixedtogether in a volume ratio of 50:50.

EXAMPLE 2

A positive active material, a positive electrode, and a cell aremanufactured according to the same method as Example 1 except that thecobalt coating is performed utilizing a wet method described in “3.cobalt coating and preparation of a final positive active material” ofExample 1. The cobalt coating process is as follows. The firstnickel-based oxide and the second nickel-based oxide are mixed togetherin a weight ratio of 7:3 and then, put in distilled water and washedtherewith, while mixed. Subsequently, 3 parts by mole of cobalt sulfate(CoSO4) based on 100 parts by mole of transition metal elements of thenickel-based oxides is slowly added thereto to perform cobalt coating.In addition, sodium hydroxide (NaOH) is slowly added thereto. Then, aproduct obtained therefrom is dried at 150° C. for 12 hours. The driedmaterial is put in a furnace and thirdly heat-treated at about 700° C.for 15 hours under an oxygen atmosphere. Subsequently, the furnace iscooled down to room temperature, obtaining a final positive activematerial in which the first and second positive active materials aremixed. In the final positive active material, the second positive activematerial having a single crystal form has protrusions and depressionsformed on the surface and an average particle diameter of about 4 μm.

COMPARATIVE EXAMPLE 1

A positive active material, a positive electrode, and a cell aremanufactured according to the same method as Example 1 except that thefirst nickel-based oxide and the second nickel-based oxide are not mixedtogether first and then, coated but are instead individually coated andthen, mixed together as described in “3. cobalt coating and preparationof a final positive active material” of Example 1. The cobalt coatingproceeds as follows. 5 parts by mole of lithium hydroxide and 3 parts bymole of cobalt oxide based on 100 parts by mole of transition metalelements are mixed together with the first nickel-based oxide and then,put in a furnace and thirdly heat-treated at about 700° C. for 15 hoursunder an oxygen atmosphere and then, cooled down to room temperature,obtaining a first positive active material. In addition, 5 parts by moleof lithium hydroxide and 3 parts by mole of cobalt oxide based on 100parts by mole of transition metal elements are mixed together with thesecond nickel-based oxide and then, put in a furnace and thirdlyheat-treated at about 850° C. for 15 hours under an oxygen atmosphereand then, cooled down to room temperature, obtaining a second positiveactive material. The cobalt-coated first positive active material andthe cobalt-coated second positive active material are mixed together ina weight ratio of 7:3, preparing a final positive active materialaccording to Comparative Example 1.

FIG. 3 is a scanning electron microscopic photograph of the secondpositive active material prepared according to Comparative Example 1.Referring to FIG. 3 , the surface of the second positive active materialof Comparative Example 1 has no protrusions and depressions on thesurface but is instead smooth and flat.

COMPARATIVE EXAMPLE 2

A positive active material and a cell are manufactured according tosubstantially the same method as Comparative Example 1 except that thethird heat-treatment of the second nickel-based oxide is performed atabout 700° C. for 15 hours under an oxygen atmosphere.

FIG. 4 is a scanning electron microscopic photograph of the secondpositive active material prepared according to Comparative Example 2.Referring to FIG. 4 , the surface of the second positive active materialof Comparative Example 2 has no protrusions and depressions but isinstead smooth and flat.

EVALUATION EXAMPLE 1 Evaluation of Surface Roughness of Positive ActiveMaterial

The positive active materials according to Examples 1 and 2 andComparative Examples 1 and 2 are measured with respect to surfaceroughness of the positive active materials through a surface roughnessmeter using atomic force microscopy (scan speed: 0.25 μm/s, non-contactmode range: 250 nm×250 nm, DME

UHV AFM). The results for the second positive active material are shownin Table 1, and the results for the first positive active material areshown in Table 2.

TABLE 1 Second Positive Example Example Comparative Comparative ActiveMaterial 1 2 Example 1 Example 2 Maximum roughness 33 23 6.9 9.9(R_(max); peak to peak height) (nm) Average roughness 3.30 1.90 0.740.97 (R_(a)) (nm) Root mean square 4.3 2.4 0.9 1.2 roughness (R_(q))(nm)

Referring to Table 1, the second positive active materials of theexamples exhibit high maximum roughness, average roughness, and rootmean square roughness, compared with the second positive active materialof the comparative examples.

TABLE 2 First Positive Comparative Active Material Example 1 Example 1Maximum roughness 7.0 9.5 (R_(max), nm) Average roughness 0.78 1.02(R_(a), nm) Root mean square 0.9 1.1 roughness (R_(q), nm)

Referring to Table 2, the surface roughness of the first positiveelectrode active material in the form of secondary particles wasanalyzed to show little difference between Example 1 of the simultaneouscoating method and Comparative Example 1 of the individual coatingmethod (where the first nickel-based oxide and the second nickel-basedoxide are coated separately).

EVALUATION EXAMPLE 2 Evaluation of Specific Surface Area

The positive active materials of Examples 1 and 2 and ComparativeExamples 1 and 2 are measured with respect to a specific surface area,and the results are shown in Table 3. The specific surface area ismeasured by using a physical and chemical adsorption phenomenon and aBrunauer-Emmett-Teller (BET) method. In other words, after measuringweights of the active materials, nitrogen is absorbed on the surface ofthe active materials, and an amount of the absorbed nitrogen gas ismeasured and used to obtain the specific surface area by using the BETmethod.

TABLE 3 BET specific surface area (m²/g) Example 1 0.3758 Example 20.3582 Comparative Example 1 0.2529 Comparative Example 2 0.2590

Referring to Table 3, the positive active materials including the firstand second positive active materials according to Examples 1 and 2exhibit an increased specific surface area, compared with the positiveactive materials of the comparative examples.

evaluation example 3 Charging/discharging Efficiency and Cycle-LifeCharacteristics

The coin half cells of Examples 1 and 2 and Comparative Examples 1 and 2are respectively charged under constant current (0.2 C) and constantvoltage (4.25 V, 0.05 C cut-off) conditions to measure charge capacityand then, paused for 10 minutes and discharged down to 3.0 V under aconstant current (0.2 C) condition to measure discharge capacity. Aratio the discharge capacity relative to the charge capacity is shown ascharge and discharge efficiency. The results are shown in Table 4.

In addition, the cells are initially charged and discharged and then, 50times charged and discharged at 1 C at 45° C. to measure the 50thdischarge capacity, and a ratio (%) of the 50th discharge capacityrelative to the initial discharge capacity is expressed as capacityretention, that is, cycle-life characteristics in Table 4.

TABLE 4 50th cycle Charge Discharge capacity capacity capacityEfficiency retention (mAh/g) (mAh/g) (%) (%) Example 1 237.7 213.7 89.995.0 Example 2 238.3 210.8 88.5 97.2 Comparative Example 1 236.9 206.387.2 94.8 Comparative Example 2 233.8 200.1 85.5 94.3

Referring to Table 4, Examples 1 and 2 exhibit increased dischargecapacity and improved charge and discharge efficiency and also, improvedhigh temperature cycle-life characteristics, compared with ComparativeExamples 1 and 2 in which the first and second positive active materialsare separately cobalt-coated and fired.

While the subject matter of this disclosure has been described inconnection with what is presently considered to be practical exampleembodiments, it is to be understood that the subject matter of thepresent disclosure is not limited to the disclosed embodiments. On thecontrary, this disclosure is intended to cover various modifications andequivalent arrangements included within the spirit and scope of theappended claims, and equivalents thereof.

Description of Symbols

100: rechargeable lithium battery 112: negative electrode 113: separator114: positive electrode 120: battery case 140: sealing member

What is claimed is:
 1. A positive active material for a rechargeablelithium battery, comprising: a first positive active material in a formof secondary particles comprising a plurality of primary particles thatare aggregated together, and a second positive active material having asingle crystal form, wherein both of the first positive active materialand the second positive active material are nickel-based positive activematerials, each of the first positive active material and the secondpositive active material is coated with cobalt, and a maximum roughness(R_(max); peak to peak height) of a surface of the second positiveactive material is greater than or equal to about 15 nm.
 2. The positiveactive material of claim 1, wherein: an average roughness (R_(a)) of thesurface of the second positive active material is greater than or equalto about 1.5 nm, and a root mean square roughness (R_(q)) of the surfaceof the second positive active material is greater than or equal to about2.0 nm.
 3. The positive active material of claim 1, wherein a BETspecific surface area of the positive active material including thefirst positive active material and the second positive active materialis about 0.3 m²/g to about 0.6 m²/g.
 4. The positive active material ofclaim 1, wherein the second positive active material has protrusions anddepressions on the surface of the second positive active material. 5.The positive active material of claim 1, wherein: an average particlediameter of the first positive active material is about 7 μm to about 25μm, and an average particle diameter of the second positive activematerial is about 1 μm to about 10 μm.
 6. The positive active materialof claim 1, wherein the first positive active material is included in anamount of about 50 wt % to about 90 wt %, and the second positive activematerial is included in an amount of about 10 wt % to about 50 wt %based on the total amount of the first positive active material and thesecond positive active material.
 7. The positive active material ofclaim 1, wherein: the first positive active material comprises acompound represented by Chemical Formula 1, and the second positiveactive material comprises a compound represented by Chemical Formula 11:Li_(a1)Ni_(x1)M¹ _(y1)M² _(1−x1−y1)O₂   Chemical Formula 1 wherein, inChemical Formula 1, 0.9≤a1≤1.8, 0.3≤x1≤1, 0≤y1≤0.7, and M¹ and M² areeach independently selected from Al, B, Ba, Ca, Ce, Co, Cr, Cu, F, Fe,Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr, and a combination thereof,Li_(a11)Ni_(x11)M¹¹ _(y11)M¹² _(1−x11−y11)O₂   Chemical Formula 11wherein, in Chemical Formula 11, 0.9≤a11≤1.8, 0.3≤x11≤1, 0≤y11≤0.7, andM¹¹ and M¹² are each independently selected from Al, B, Ba, Ca, Ce, Co,Cr, Cu, F, Fe, Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr, and acombination thereof.
 8. A method of preparing a positive active materialfor a rechargeable lithium battery, the method comprising: mixing afirst nickel-based hydroxide and a lithium raw material together andperforming a first heat-treatment to prepare a first nickel-based oxidein a form of secondary particles comprising a plurality of primaryparticles that are aggregated together; mixing a second nickel-basedhydroxide together with a lithium raw material and performing a secondheat-treatment to prepare a second nickel-based oxide in a singlecrystal form, and mixing the first nickel-based oxide, the secondnickel-based oxide, and a cobalt compound together and performing athird heat-treatment to coat the first nickel-based oxide and the secondnickel-based oxide with cobalt, and obtaining a final positive activematerial including the first positive active material and the secondpositive active material.
 9. The method of claim 8, wherein the firstnickel-based hydroxide and the second nickel-based hydroxide are eachindependently represented by Chemical Formula 21:Ni_(x21)M²¹ _(y21)M²² _(1−x21−y21)(OH)₂   Chemical Formula 21 wherein,in Chemical Formula 21, 0.3≤x21≤1, 0≤y21≤0.7, and M²¹ and M²² are eachindependently selected from Al, B, Ba, Ca, Ce, Co, Cr, Cu, F, Fe, Mg,Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr, and a combination thereof. 10.The method of claim 8, wherein: the mixing of the first nickel-basedhydroxide together with the lithium raw material is performed in a ratioof a mole number of lithium in the lithium raw material relative to amole number of elements excluding hydrogen and oxygen included in thefirst nickel-based hydroxide of greater than or equal to about 0.9 andless than or equal to about 1.2, and the mixing of the secondnickel-based hydroxide together with the lithium raw material isperformed in a ratio of a mole number of lithium in the lithium rawmaterial relative a mole number of elements excluding hydrogen andoxygen included in the second nickel-based hydroxide of greater than orequal to about 0.9 and less than or equal to about 1.2.
 11. The methodof claim 8, wherein the first heat-treatment is performed at about 600°C. to about 900° C.
 12. The method of claim 8, wherein the firstheat-treatment is performed for about 5 hours to about 20 hours.
 13. Themethod of claim 8, wherein the second heat-treatment is performed atabout 800° C. to about 1100° C.
 14. The method of claim 8, wherein thesecond heat-treatment is performed for about 5 hours to about 20 hours.15. The method of claim 8, wherein: the preparing of the secondnickel-based oxide comprises obtaining the second nickel-based oxide inthe single crystal form by mixing the second nickel-based hydroxidetogether with the lithium raw material, secondarily heat-treating theresultant mixture, and pulverizing a resultant therefrom.
 16. The methodof claim 8, wherein: the mixing of the first nickel-based oxide and thesecond nickel-based oxide is performed by mixing the first nickel-basedoxide and the second nickel-based oxide together in a weight ratio ofabout 9:1 to about 5:5.
 17. The method of claim 8, wherein: the mixingof the first nickel-based oxide, the second nickel-based oxide, and thecobalt compound together is performed so that cobalt included in thecobalt compound is included in an amount of about 0.01 parts by mole toabout 7 parts by mole, based on 100 parts by mole of the total amount ofelements excluding lithium and oxygen in the first nickel-based oxideand the second nickel-based oxide.
 18. The method of claim 8, whereinthe third heat-treatment is performed at about 650° C. to about 900° C.19. The method of claim 8, wherein the third heat-treatment is performedfor about 5 hours to about 30 hours.
 20. A rechargeable lithium batterycomprising: a positive electrode including the positive active materialof claim 1, a negative electrode, and an electrolyte.